The present invention relates to a pressure washer which pumps liquid supplied from an external source through a spray nozzle at high pressure. The pressure washer may be in a standing form with an elongate hose leading to a spray lance or spray nozzle, or it can be a portable, hand held unit. The liquid may be pumped at a pressure in the vicinity of 1,000 psi. The pathway through which the liquid is pumped is typically selectively openable to permit the liquid to be sprayed from the spray nozzle and closable to halt the spray of liquid. In a portable version, the pump is typically operated when liquid spraying is required. The pump is thus switched on and off as needed by an electric switch. On the other hand, in a standing form of the pressure washer, the means which pumps the liquid typically operates continuously whether the liquid pathway is opened or closed, requiring protection of the system against damage when the liquid pathway is closed. One known technique for protecting the continuous system comprises selective bypassing of pumped liquid back to the pump inlet when the liquid outlet pathway is closed. A valve controls the bypass arrangement to permit bypass recirculation at a lower pressure to prevent overheating due to recirculation of high pressure. A valve for the bypass arrangement can desirably control that pressure to maintain its desired level.
Often the pressure washer is used to pump liquid, and particularly water at high pressure. Where the water is used for cleaning purposes, it may be desirable to mix another liquid, like a detergent or the like chemical, with the water, and appropriate means are desirable for controllable mixing of the additional liquid with the water being pumped. Various means for supplying an additional liquid into the main liquid flow are known in the art.
Various pressure washers are known. They use various pump arrangements for delivering liquid under pressure. Some known pressure washers use a piston pump, where the piston is caused to reciprocate by various means. Although one piston would pump the liquid, it is preferred to have a number of pistons. This provides the optimum balance of speed, torque, bearing life, valve design, and the like, to provide the desired flow rate and high efficiency. This also produces a generally more continuous spray. Therefore, a plurality of pistons pump the liquid and appropriate means sequence the piston operation.
In one known arrangement, the piston in such a pump is an articulated piston But the articulation connection gives rise to side thrusts and loss of efficiency, as well as being more complex. In another known arrangement, the pistons are not articulated. Instead, a swash plate rotates past the pistons to reciprocate them in sequence for pumping liquid.